This invention relates to a vital inverter driver, especially of the type utilized in railroad signalling installations where the conversion to a.c. power from a d.c. battery source must be made within a tight frequency tolerance. Because of the vitality required of the overall railroad signalling installation and thus the a.c. power devices associated therewith, such as, for example, a.c. track circuits, communication/carrier equipment, and other phase selective devices; such vitality is also required of the inverter driver whereby, in the event of a drifting away from the tolerance of the frequency, the output of the vital inverter driver is immediately shut down. Some typical vital inverter drivers have used a vibrating reed-type oscillator by which is established the frequency of the output voltage. Though accurate, such vibrating reed-type oscillators are expensive, difficult to obtain, and are limited in application to only the one selected frequency. Additionally, vital power inverters which have used this vibrating reed-type oscillator have relied on the integrity of the frequency output as a means of shutting down the devices to which the inverter is supplying power; that is, any frequency drift resulted in a shutdown of the powered phase selective device and not of the inverter itself.